The present invention relates generally to the field of manufacturing forged aluminum connecting rods, and more particularly to an improved high performance aluminum connecting rod and a method for manufacturing such a connecting rod which is capable of carrying particularly high compressive loads with a substantially reduced deformation resulting from such high compressive loads, the improved high performance aluminum connecting rod of the present invention finding particular application in the field of high performance racing engines.
Internal combustion engines used in automobiles typically have pistons which reciprocate in cylinders contained in an engine block, with the pistons driving connecting rods which in turn drive a single geometrically complex crankshaft, causing it to rotate within the engine block. The pistons are cylindrical and are hollowed out at the end facing the crankshaft. Wrist pins extending diametrically through the pistons near the open end thereof are used to connect each piston to one end of a connecting rod which has a cylindrical wrist pin bore located therein, which end is referred to as the pin end of the connecting rod.
The other end of each connecting rod is connected to the crankshaft at a particular cylindrical bearing location on the crankshaft. This end of the connecting rod, which is referred to as the crank end, also has a cylindrical bearing housing bore located therein. Typically, an annular bearing is located between the bearing housing bore at the crank end of the connecting rod and the cylindrical bearing location on the crankshaft.
The crank end of the connecting rod is split into two segments, with the split being defined by a plane which lies at an angle to the longitudinal axis of the connecting rod (the axis extending between the pin end and the crank end of the connecting rod), which plane divides the bearing housing bore at the crank end of the connecting rod in half. The end of the main segment of the connecting rod (the segment which includes the pin end) which is opposite the pin end is referred to as the fork of the connecting rod, since it includes half of the bearing housing bore at the crank end of the connecting rod.
The smaller segment of the connecting rod is referred to as the cap or the bearing cap, and it includes the other half of the bearing housing bore at the crank end of the connecting rod. The cap typically is retained in position on the fork by two bolts located on opposite sides of the cap and fork. The bolts extend into holes located in the, cap, and are screwed into threaded apertures located in the fork.
As is generally well known in the art, the development of the reciprocating internal combustion engine has resulted in ever-smaller engines which are capable of generating an ever-increasing amount of horsepower, at ever-higher RPM""s. It will be appreciated by those skilled in the art that with the increasing power has come increased stresses which are placed on the various parts of the engine, including increased stresses placed on the connecting rods and bearings. These advancements in engine technology have resulted in a demand for connecting rods which are both lighter and stronger than previous generations of connecting rods.
Nowhere is this demand for improved connecting rod strength and lightness more important than in the field of auto racing, including gas, alcohol, and nitro engines which are used in drag racing. These engines are subjected to incredible loads, and this is particularly true with respect to the ultra high combustion pressure loads that the connecting rods are subjected to. The loads are compressive in nature, and whenever the yield strength of the connecting rods is exceeded, they will be damaged or destroyed. Typical damage which can result from these stresses includes bending of the connecting rod beam (the portion of the connecting rod extending between the pin end and the fork), elongation of the wrist pin bore, and deformation of the bearing housing bore, particularly in the cap of the connecting rod.
The materials which are used to make high performance connecting rods are almost exclusively high tech aluminum alloys, which possess both relatively light weight and the requisite high degree of material strength. Aluminum alloys have been used for high performance engines such as those used in drag racing for thirty years, with the first alloy being used being 2014-T6 aluminum alloy, which has a compressive yield strength of 58 ksi. More recently, 7075-T6 aluminum alloy has been used, which has a substantially higher compressive yield strength of 72 ksi. Neither of these alloys currently possesses sufficient compressive yield strength to withstand the compression loads of current drag racing engine technology.
In addition, the manufacture of high performance aluminum connecting rods from aluminum material must be of a nature which enhances, and in no way reduces, the material strength characteristics of the aluminum material. To understand this, it is necessary to briefly describe the manufacture of connecting rods. There are three primary methods of manufacturing connecting rods: casting, powder metal manufacturing rolled plate stock, and foregoing. Each of these methods will be briefly described below.
Casting involves heating the metal alloy which will be used until it is molten, and pouring the molten metal alloy into xe2x80x9csprewsxe2x80x9d which are connected to xe2x80x9crunnersxe2x80x9d which channel the molten metal alloy into a multiple cavity mold. The molten metal alloy fills the mold, and vents through xe2x80x9crisers.xe2x80x9d After cooling, the metal casting is separated from the mold, the sprews, runners, and risers are removed, and the casting is machined.
Casting is a relatively low-cost manufacturing process, and results in a product with tight dimensional tolerances. Unfortunately, casting has several disadvantages, including relatively low strength characteristics, an absence of grain flow strength, and possible porosity of the cast article.
These negative characteristics make casting an unacceptable manufacturing process for high performance aluminum connecting rods.
Powder metal manufacturing involves placing a mixture of metal powder into a preformed die, and then compressing the powder in the die with a compacting press to mechanically bond the metal powder into a preform. The preform is then sintered to chemically bond the powder in the preform, and the preform is then heated and forged in a press. The resulting article is then machined to finish it.
Powder metal manufacturing results in high tolerances and uniform material composition of the finished article. Unfortunately, powder metal manufacturing is a relatively high cost operation, has a lack of grain flow strength, and results in a product which has relatively low tensile strength and impact strength. These negative characteristics make powder metal manufacturing an unacceptable manufacturing process for high performance aluminum connecting rods.
The rolled plate stock approach includes machining to form a billet connecting rod. However, the rod has relatively low strength and no grain flow strength.
Forging involves heating a piece of bar stock or rod stock (referred to as a forge slug) to a forging temperature, placing the heated forge slug into a two piece forging die defining a cavity within, and forging the forge slug using a forging press or forging hammers with multiple impacts. Excess material results in flashing extending between the two dies. This flashing is removed by placing the forging into a trim die, which is placed in a punch press to trim the flashing from the forged article. The part is then machined to finished dimensions.
Forging can be somewhat labor intensive due to the amount of machining which is necessary to produce the finished article, but it produces a product which has strength characteristics which are superior to those produced by casting rolled plate stock or powder metal manufacturing. This results from the fact that the natural grain of rolled metal stock is maintained in the finished product. The natural grain extends along the full length of the forged rod, from one end where it is expanded. The natural grain is not truncated, as is the case with a billet. As such, it will be apparent to those skilled in the art that forging is the preferred method of manufacture for high performance aluminum connecting rods.
It is accordingly the primary objective of the present invention that it provide an improved high performance aluminum connecting rod made of an improved material capable of withstanding the high compressive loads of current drag racing technology. The material used by the improved high performance aluminum connecting rod of the present invention must have lightweight construction which is comparable to presently known high performance aluminum materials such as 7075-T6, while possessing substantially enhanced compression yield strength characteristics as compared to such presently known materials. In addition, this improved material must retain or enhance all of the other favorable material characteristics of such presently known materials.
The improved high performance aluminum connecting rod of the present invention must be susceptible of manufacture by conventional forging techniques, with the forged part being readily machinable to the required finished dimensions. It is also necessary that the high performance characteristics of the aluminum material used to make the improved high performance aluminum connecting rod of the present invention not be adversely affected in the forging or machining operations. Most importantly, it is an objective of the improved high performance aluminum connecting rod of the present invention that it have sufficiently improved compression yield strength so as to hold both its dimensional length and the dimensional roundness of the wrist pin bore and the bearing housing bore.
The improved high performance aluminum connecting rod of the present invention must be of a construction which is both durable and long lasting, and it must remain within dimensional specifications throughout an extended operating lifetime. In order to enhance the market appeal of the present invention, it should also be of relatively inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives of the improved high performance aluminum connecting rod of the present invention be achieved without incurring any substantial relative disadvantage.
The disadvantages and limitations of the background art discussed above are overcome by the improved high performance aluminum connecting rod of the present invention. With this invention, an improved high performance aluminum alloy is used as the material used to fabricate the improved high performance aluminum connecting rod of the present invention. This material, which is a variant of the aluminum alloy taught in U.S. Pat. No. 5,221,377, to Hunt, Jr., et al., assigned of record to the Aluminum Company of America (Alcoa), has an improved combination of strength, density, and toughness, with a yield strength which is substantially higher than aluminum alloys such as 7075-T6 which have previously been used for connecting rods. U.S. Pat. No. 5,221,377 is hereby incorporated herein by reference.
This improved high performance aluminum alloy is obtained in extruded bar stock which is suitable for forging, either as rectangular bar stock or as round bar stock. The improved high performance aluminum alloy bar stock is of a suitable size and is cut to an appropriate length. The segment of high performance aluminum alloy bar stock is heated to a temperature which was determined after extensive testing. The heated segment of high performance aluminum alloy bar stock is then forged in a blocker die to rough out the shape of a connecting rod. The rough part is forged in a finisher die, and is then cooled.
The flashing resulting from the foregoing process is cleaned off of the forged part in a trim die. The forged part is then heat treated and etched, as is conventional in the art, after which it is penetrant inspected and ultrasonically tested. The forged part is then ball burnished, completing the first portion of the manufacturing process.
The preferred embodiment of the improved high performance aluminum connecting rod of the present invention will be described in detail, but first the high performance aluminum alloy from which it is manufactured will be described in detail. As mentioned above, this high performance aluminum alloy is a variant of the aluminum alloy taught in Alcoa""s U.S. Pat. No. 5,221,377, which has previously been incorporated herein by reference. Prior to the development leading up to the improved high performance aluminum connecting rod of the present invention, this aluminum alloy has not been known to be suitable for the fabrication of connecting rods.
The preferred high performance aluminum alloy used to make the improved high performance aluminum connecting rod of the present invention is an alloy available from Aluminum Company of America xe2x80x9cAlcoaxe2x80x9d as 7055-T77511, also referred to as HP007. This aluminum alloy has been available from Alcoa for some time, but never made into extruded bar stock for forging. By working with Alcoa, the inventors of the invention described herein obtained 7055-T77511 aluminum alloy from Aluminum Company of America xe2x80x9cAlcoaxe2x80x9d in extruded bar stock suitable for forging. The 7055-T77511 aluminum alloy used to fabricate the improved high performance aluminum connecting rod of the present invention is formulated (by weight) as follows: drilled into the fork of the connecting rod. The holes in the fork of the connecting rod are threaded.
The cap is assembled onto the fork of the connecting rod using lubricated rod bolts, which are torqued to the proper degree. Bearing tangs and an oil-retaining aperture for a bearing retaining pin are also machined into the bearing housing bore, and an oil passage is machined into the wrist pin bore. The bearing housing bore is then rough bored, finish bored, and honed to the proper size. On the pin end of the connecting rod, the wrist pin bore is bored and honed to the proper size.
The improved high performance aluminum connecting rod of the present invention has been found to possess a number of significant advantages over previously-known connecting rods, with absolutely no attendant disadvantages. The improved high performance aluminum connecting rod of the present invention has a substantially increased compressive yield strength as well as a substantially increased compressive yield strength as well as a substantially increased tensile yield strength. Under testing in drag racing engines generating in excess of 6,000 horsepower, the improved high performance aluminum connecting rod of the present invention has been found to have a compressive yield strength of 25 percent greater than 7075-T6.
The increase in the compressive yield strength of the improved high performance aluminum connecting rod of the present invention effectively prevents bending of the connecting rod beam, elongation of the wrist pine bore, and a marked deterioration in the roundness of the bearing housing bore. As compared to a conventional high performance aluminum connecting rod made of 7075-T6, which typically experiences a compression of 0.004 to 0.005 inch after one run at full load, the improved high performance aluminum connecting rod of the present invention remains within 0.001 inch of its original dimension after one run at full load.
The wrist pin bore of the improved high performance aluminum connecting rod of the present invention remains within 0.0003 to 0.0005 inch of its original dimension after one run at full load, as compared to the wrist pin bore of a conventional high performance aluminum connecting rod, which loses roundness of 0.002 to 0.003 inch after one run at full load. Finally, the bearing housing bore of the improved high performance aluminum connecting rod of the present invention remains within 0.0003 to 0.0005 inch of its original dimension after one run at full load, as compared to the bearing housing bore of a conventional high performance aluminum connecting rod, which loses roundness of 0.001 to 0.0015 inch after one run at full load. These increases in strength and performance are particularly significant in view of the fact that conventional high performance aluminum connecting rod made of 7075-T6 were the state of the art prior to the development of the improved high performance aluminum connecting rod of the present invention.
It may therefore be seen that the present invention teaches an improved high performance aluminum connecting rod made of an improved material capable of withstanding the high compressive loads of current drag racing engine technology. The material used by the improved high performance aluminum connecting rod of the present invention has lightweight construction which is comparable to presently known high performance aluminum materials such as 7075-T6, while possessing substantially enhanced compression yield strength characteristics as compared to such presently known materials. In addition, this improved material also retains or enhances all of the other favorable material characteristics of such presently known materials.
The improved high performance aluminum connecting rod of the present invention is readily susceptible of manufacture by conventional forging techniques, with the forged part being readily machinable to the required finished dimensions. The high performance characteristics of the aluminum material used to make the improved high performance aluminum connecting rod of the present invention are not adversely affected in either the forging or machining operations. Most importantly, the improved high performance aluminum connecting rod of the present invention has sufficiently improved compression yield strength so as to hold both its dimensional length and the dimensional roundness of the wrist pin bore and the bearing housing bore.
The improved high performance aluminum connecting rod of the present invention is of a construction which is both durable and long lasting, and which will remain within dimensional specifications throughout an extended operating lifetime. The improved high performance aluminum connecting rod of the present invention is also of relatively inexpensive construction to enhance its market appeal and to thereby afford it the broadest possible market. Finally, all of the aforesaid advantages and objectives of the improved high performance aluminum connecting rod of the present invention are achieved without incurring any substantial relative disadvantage.